Chapter: Cell Reproduction

Chapter: Cell Reproduction

Table of ContentsTable of Contents

Section 3: DNA

Section 1: Cell Division and Mitosis

Section 2: Sexual Reproduction and Meiosis

• Many organisms start as just one cell.• That cell divides and becomes two, two

become four, four become eight, an so on.• Many-celled

organisms, including you, grow because cell division increases the total number of cells in an organism.

Why is cell division important?

Cell Division and MitosisCell Division and Mitosis

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• Even after growth stops, cell division is still important.

• Every day, billions of red blood cells in your body wear out and are replaced.

• During a few seconds, your bone marrow produced about six million red blood cells.

Why is cell division important?


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• Cell division is important to one-celled organisms, too—it’s how they reproduce themselves.

• A living organism has a live cycle.• A life cycle begins

with the organism’s formation, is followed by growth and development, and finally ends in death.

• Individual cells also have life cycles.

The Cell Cycle


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Click image to view movie.

• The cell cycle is a series of events that takes place from one cell division to the next.

• The time is takes to complete a cell cycle is not the same in all cells.

Length of Cycle


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• Most of the life of any eukaryotic cell—a cell with a nucleus—is spent in a period of growth and development called interphase.

• Cells in your body that no longer divide, such as nerve and muscle cells, are always in interphase.

Interphase


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• An actively dividing cell, such as a skin cell, copies its hereditary material and prepares for cell division during interphase.

• Before a cell divides, a copy of the hereditary material must be made so that each of the two new cells will get a complete copy.

• Each cell needs a complete set of hereditary material to carry out life functions.

Interphase


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Interphase


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• After interphase, cell division begins.

• The nucleus divides, and then cytoplasm separates to form two new cells.

• Mitosis (mi TOH sus) is the process in which the nucleus divides to form two identical nuclei.

• Each new nucleus also is identical to the original nucleus.

Mitosis


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• Mitosis is described as a series of phases, or steps.

• The steps of mitosis in order are named prophase, metaphase, anaphase, and telophase.

• A chromosome (KROH muh sohm) is a structure in the nucleus that contains hereditary material.

Steps of Mitosis


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• During interphase, each chromosome duplicates.

Steps of Mitosis


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Steps of Mitosis


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• When the nucleus is ready to divide, each duplicated chromosome coils tightly into two thickened, identicalstrands called chromatids.

• During prophase, the pairs of chromatids are fully visible when viewed under a microscope.

• The nucleolus and the nuclear membrane disintegrate.

Steps of Mitosis


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• Two small structures called centrioles (SEN tree olz) move to opposite ends of the cell.

• Between the centrioles, threadlike spindle fibers begin to stretch across the cell.

• Plant cells also form spindle fibers during mitosis but do not have centrioles.

Steps of Mitosis


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• In metaphase, the pairs of chromatids line up across the center of the cell.

• The centromere of each pair usually becomes attached to two spindle fibers—one from each side of the cell.

Steps of Mitosis


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• In anaphase, each centromere divides and the spindle fibers shorten.

• Each pair of chromatids separates, and chromatids begin to move to opposite ends of the cell.

Steps of Mitosis


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• The separated chromatids are now called chromosomes.

• In the final step, telophase, spindle fibers start to disappear, the chromosomes start to uncoil, and a new nucleus forms.

Steps of Mitosis


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• For most cells, after the nucleus has divided, the cytoplasm separates and two new cells are formed.

Division of the Cytoplasm


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• In animal cells, the cell membrane pinches in the middle, like a balloon with a string tightened around it, and the cytoplasm divides.

Click box to view movie.

• In plant cells, the appearance of a cell plate tells you that the cytoplasm is being divided.

Division of the Cytoplasm


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• New cell walls form along the cell plate, and new cell membranes develop inside the cell walls.

• Following division of the cytoplasm, most new cells begin the period of growth, or interphase, again.

Results of Mitosis


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• Mitosis is the division of the nucleus, and it produces two new nuclei that are identical to each other and the original nucleus.

Results of Mitosis


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• Each new nucleus has the same number and type of chromosomes.

• Every cell in your body, except sex cells, has a nucleus with 46 chromosomes—23 pairs.

• Each of the trillions of cells in your body, except sex cells, has a copy of the same hereditary material.

Results of Mitosis


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• All of your cells use different parts of the same hereditary material to become different types of cells.

• Cell division allows growth and replaces worn out or damaged cells.

Results of Mitosis


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• If you cut yourself, the wound heals because cell division replaces damaged cells.

• Another way some organisms use cell division is to produce new organisms.

• Reproduction is the process by which an organism produces others of its same kind.

Asexual Reproduction


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• Among living organisms, there are two types of reproduction—sexual and asexual.

• Sexual reproduction usually requires two organisms.

• In asexual reproduction, a new organism (sometimes more than one) is produced from one organism.

Asexual Reproduction


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• The new organism will have hereditary material identical to the hereditary material of the parent organism.

• Organisms with eukaryotic cells asexually reproduce by cell division.

Cellular Asexual Reproduction


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• However, bacteria do not have a nucleus so they can’t use mitosis.

• Instead, bacteria reproduce asexually by fission.

• During fission, an organism whose cells do not contain a nucleus copies its genetic material and then divides into two identical organisms.

• Budding is a type of asexual reproduction made possible because of cell division.

Budding and Regeneration


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• When the bud on the adult becomes large enough, it breaks away to live on its own.

• Some organisms can regrow damaged or lost body parts.

Budding and Regeneration


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• Regeneration is the process that uses cell division to regrow body parts.

• Sponges, planaria, sea stars, and some other organisms can use regeneration for asexual reproduction.

Section CheckSection Check

Question 1

Answer

Many-celled organisms grow by _______.

The answer is cell division. Cell division increases the total number of cells in an organism.

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Question 2

What is the longest part of the cell cycle?

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Answer

The answer is interphase. Interphase is the period of growth and development in a eukaryotic cell.

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Question 3

Answer

_______ is the process in which the nucleus divides to form two identical nuclei.

The answer is mitosis. Each new nucleus that is produced is identical to the original nucleus.

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Sexual Reproduction• During sexual reproduction, two sex cells,

sometimes called an egg and a sperm, come together.

• Sex cells are formed from cells in reproductive organs.

• Sperm are formed in the male reproductive organs.

Sexual Reproduction and MeiosisSexual Reproduction and Meiosis

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• Eggs are formed in the female reproductive organs.

Sexual Reproduction• The joining of an egg and a sperm is called

fertilization, and the cell that forms is called a zygote (ZI goht).

• Following fertilization, cell division begins. A new organism with a unique identity develops.


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Diploid Cells• A typical human body cell has 46

chromosomes.• Each chromosome has a mate that is similar

to it in size and shape and has similar DNA.


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• Human body cells have 23 pairs of chromosomes.

• When cells have pairs of similar chromosomes, they are said to be diploid (DIH ployd).

Haploid Cells• Because sex cells do not have pairs of

chromosomes, they are said to be haploid (HA ployd).


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• They have only half the number of chromosomes as body cells. Haploid means “single form.”

• Human sex cells have only 23 chromosomes—one from each of the 23 pairs of similar chromosomes.

Meiosis and Sex Cells• A process called meiosis (mi OH sus)

produces haploid sex cells.


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Meiosis and Sex Cells


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• Meiosis ensures that the offspring will have the same diploid number as its parent.



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• After two haploid sex cells combine, a diploid zygote is produced that develops into a new

diploid organism.

Meiosis and Sex Cells• During meiosis, two divisions of the nucleus

occur. These divisions are called meiosis I and meiosis II.


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• The steps of each division have names like those in mitosis and are numbered for the division in which they occur.

Meiosis I• Before meiosis begins, each chromosome is

duplicated, just as in mitosis.


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Meiosis I


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• When the cell is ready for meiosis, each duplicated chromosome is visible under the microscope as two chromatids.

Meiosis I• The events of prophase I are similar to those

of prophase in mitosis.


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• In meiosis, each duplicated chromosome comes near its similar duplicated mate.

• In mitosis they do not some near each other.

Meiosis I• In metaphase I, the pairs of duplicated

chromosomes line up in the center of the cell.


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• The centromere of each chromatid pair becomes attached to one spindle fiber, so the chromatids do not separate in anaphase I.

Meiosis I• In anaphase I, the two

pairs of chromatids of each similar pair move away from each other to opposite ends of the cell.


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• Each duplicated chromosome still has two chromatids.

Meiosis I

• In telophase I, the cytoplasm divides, and two new cells form.


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• Each new cell has one duplicated chromosome from each similar pair.

Meiosis II• The two cells formed during meiosis I now

begin meiosis II.


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Meiosis II


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• The chromatids of each duplicated chromosome will be separated during this division.

Meiosis II• In prophase II, the

duplicated chromosomes and spindle fibers reappear in each new cell.


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Meiosis II• In metaphase II, the

duplicated chromosomes move to the center of the cell.


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• Unlike what occurs in metaphase I, each centromere now attaches to two spindle fibers instead of one.

Meiosis II• The centromere divides

during anaphase II, and the chromatids separate and move to opposite ends of the cell.


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• Each chromatid now is an individual chromosome.

Meiosis II• As telophase II begins,

the spindle fibers disappear, and a nuclear membrane forms around the chromosomes at each end of the cell.


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• When meiosis II is finished, the cytoplasm divides.

Summary of Meiosis


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• Remember that meiosis produces haploid sex cells.

Click box to view movie.

• This animation will help you further understandmeiosis.

Mistakes of Meiosis• Meiosis occurs many times in reproductive

organs.


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• Mistakes can produce sex cells with too many or too few chromosomes.

Mistakes of Meiosis

• Sometimes, zygotes produced from these sex cells die.


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• If the zygote lives, every cell in the organism that grows from that zygote usually will have the wrong number of chromosomes.

• Organisms with the wrong number of chromosomes may not grow normally.


Question 1

Answer

_______ is the joining of an egg and a sperm.

The answer is fertilization. Fertilization occurs during sexual reproduction.

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22Section CheckSection Check

Question 2

_______ and _______ are the two types of cells your body forms.

A. body and sexB. egg and spermC. meiosis and mitosisD. zygote and embryo


Answer

The answer is A. Egg cells and sperm cells are both types of sex cells.


Question 3

Which best describes the sperm and egg?


A. typical body cellsB. haploid cells

C. zygotesD. diploid cells


Answer

The answer is B. Sperm and eggs are sex cells that do not have pairs of chromosomes.

What is DNA?

• A cell uses a code in its hereditary material. The code is a chemical called deoxyribonucleic (dee AHK sih ri boh noo klay ihk) acid, or DNA.

• It contains information for an organism’s growth and function.

DNADNA

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What is DNA?• DNA is stored in cells that have a nucleus.• When a cell divides, the DNA code is copied

and passed to the new cells.

DNADNA

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• In this way, new cells receive the same coded information that was in the original cell.

Discovering DNA

• Since the mid-1800s, scientists have known that the nuclei of cells contain large molecules called nucleic acids.

DNADNA

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• By 1950, chemists had learned what nucleic acid DNA was made of, but they didn’t understand how the parts of DNA were arranged.

DNA’s Structure

• In 1952, scientist Rosalind Franklin discovered that DNA is two chains of molecules in a spiral form.

DNADNA

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• By using an X-ray technique, Dr. Franklin showed that the large spiral was probably made up of two spirals.

• In 1953, scientists James Watson and Francis Crick made a model of a DNA molecule.

A DNA Model• According to the Watson

and Crick DNA model, each side of the ladder is made up of sugar-phosphate molecules.

DNADNA

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• Each molecule consists of the sugar called deoxyribose (dee AHK sih ri bohs) and a phosphate group.

A DNA Model• The rungs of the ladder are made up of other

molecules called nitrogen bases.

DNADNA

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A DNA Model

DNADNA

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• Four kinds of nitrogen bases are found in DNA—adenine(A duh neen), guanine (GWAH neen), cytosine (SI tuh seen), and thymine (THI meen).

A DNA Model

DNADNA

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• The bases are represented by the letters A, G, C, and T.

A DNA Model• The amount of cytosine

in cells always equals the amount of guanine, and the amount of adenine always equals the amount of thymine.

DNADNA

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• Adenine always pairs with thymine, and guanine always pairs with cytosine.

Copying DNA• When chromosomes are duplicated before

mitosis or meiosis, the amount of DNA in the nucleus is doubled.

DNADNA

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• The two sides of DNA unwind and separate.

Copying DNA

DNADNA

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• Each side then becomes a pattern on which a new side forms.

• The new DNA has bases that are identical to those of the original DNA and are in the same order.

Genes

• Most of your characteristics, such as the color of your hair, your height, and even how things taste to you, depend on the kinds of proteins your cells make.

DNADNA

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• DNA in your cells stores the instructions for making these proteins.

Genes

DNADNA

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• Proteins build cells and tissues or work as enzymes.

• The instructions for making a specific protein are found in a gene which is a section of DNA on a chromosome.

Genes

DNADNA

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• Each chromosome contains hundreds of genes.

• Proteins are made of chains of hundreds or thousands of amino acids.

• The gene determines the order of amino acids in a protein.

• Changing the order of the amino acids makes a different protein.

Making Proteins

DNADNA

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• Genes are found in the nucleus, but proteins are made on ribosomes in cytoplasm.

• The codes for making proteins are carried from the nucleus to the ribosomes by another type of nucleic acid called ribonucleic acid, or RNA.

Ribonucleic Acid

DNADNA

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• RNA is made in the nucleus on a DNA pattern. However, RNA is different from DNA.

• RNA is like a ladder that has all its rungs sawed in half.

• RNA has the bases A, G, and C like DNA but has the uracil (U) instead of thymine (T).

• The sugar-phosphate molecules in RNA contain the sugar ribose, not deoxyribose.

Ribonucleic Acid

DNADNA

33

• The three main kinds of RNA made from DNA in a cell’s nucleus are messenger RNA (mRNA), ribosomal (rRNA), and transfer RNA (tRNA).

• Protein production begins when mRNA moves into the cytoplasm. There, ribosomes attach to it.

Ribonucleic Acid

DNADNA

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• Ribosomes are made of rRNA.• Transfer RNA molecules in the cytoplasm

bring amino acids to these ribosomes.• Inside the

ribosomes, three nitrogen bases on the mRNA temporarily match with three nitrogen bases on the tRNA.

Ribonucleic Acid

DNADNA

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• The same thing happens for the mRNA and another tRNA molecule.

• The amino acids that are attached to the two tRNA molecules bond. This is the beginning of a protein.

Ribonucleic Acid

DNADNA

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• The code carried on the mRNA directs the order in which the amino acids bond.

• After a tRNA molecule has lost its amino acid, it can move about the cytoplasm and pick up another amino acid just like the first one.

• The ribosome moves along the mRNA.• New tRNA molecules with amino acids

match up and add amino acids to the protein molecule.

Controlling Genes

DNADNA

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• In many-celled organisms like you, each cell uses only some of the thousands of genes that it has to make proteins.

• Each cell uses only the genes that direct the making of proteins that it needs.

• For example, muscle proteins are made in muscle cells but not in nerve cells.

Controlling Genes

DNADNA

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• Cells must be able to control genes by turning some genes off and turning other genes on.

• Sometimes the DNA is twisted so tightly that no RNA can be made.

• Other times, chemicals bind to the DNA so that it cannot be used.

• If the incorrect proteins are produced, the organism cannot function properly.

Mutations

DNADNA

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• If DNA is not copied exactly, the proteins made from the instructions might not be made correctly.

• These mistakes, called mutations, are any permanent change in the DNA sequence of a gene or chromosome of a cell.

• Outside factors such as X rays, sunlight, and some chemicals have been known to cause mutations.

Results of a Mutation

DNADNA

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• Genes control the traits you inherit.

• Without correctly coded proteins, an organism can’t grow, repair, or maintain itself.

• A change in a gene or chromosome can change the traits of an organism.


DNADNA

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• If the mutation occurs in a body cell, it might or might not be life threatening to the organism.

• If a mutation occurs in a sex cell, then all the cells that are formed from that sex cell will have that mutation.

• Mutations add variety to a species when the organism reproduces.


DNADNA

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• Many mutations are harmful to organisms, often causing their death.

• Some mutations do not appear to have any effect on the organism, and some can even be beneficial.


Question 1

Answer

_______ is the chemical code that is stored in the cell’s hereditary material.

The answer is deoxyribonucleic acid, or DNA. DNA contains information for an organism’s growth and function.

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Question 2

Which is not a nitrogen base of DNA?

A. adenine B. guanine C. kerosene D. thymine


Answer

The answer is C. The forth kind of nitrogen base is cytosine.


Question 3This illustration represents the production of _______?

A. DNAB. genesC. proteinsD. RNA


Answer

The answer is C. Cells need DNA, RNA, and amino acids to make proteins.

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Chapter: Cell Reproduction

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